1. Field of the Invention
The present invention relates to a nematic liquid crystal composition and a liquid crystal display element. More particularly, it relates to a liquid crystal composition suitably usable for a liquid crystal display element using a super twisted nematic (STN) display mode, and a liquid crystal display element using this liquid crystal composition.
2. Description of the Related Art
As liquid crystal display modes, there have been proposed a twisted nematic (TN) mode, a supertwisted nematic (STN) mode, an active matrix mode, etc., which have been practically utilized one after another. Among these modes, the STN mode proposed by T. J. Scheffer et al. [Appl. Phys. Lett., 45 (10), 1021 (1984)] wherein the alignment of liquid crystal molecules in the upper and lower substrates is twisted at an angle of 180xc2x0 to 270xc2x0 has been adopted in liquid crystal display elements for personal computers and the like.
In recent years, the STN mode has been also used in liquid crystal display elements for potable information terminals such as electric pocket notebooks and small size note type personal computers which are often used outdoors.
The general characteristics required for the STN mode have been the following paragraphs (1) to (4), but for a liquid crystal material usable for the STN mode of outdoor use, the following characteristics of the paragraphs (5), (6) and (7) are further required.
(1) To make a steepness (xcex3) in a voltage-transmittance curve (a V-T curve) of a liquid crystal composition close to 1 as much as possible, for the purpose of attaining a high contrast of a liquid crystal display element.
(2) To lower a viscosity (xcex7) of the liquid crystal composition as much as possible, for the purpose of shortening a response time of the liquid crystal display element.
(3) To permit taking a suitable optical anisotropy (xcex94n) of the liquid crystal composition in accordance with a cell thickness of the liquid crystal display element, for the purpose of attaining an optimum contrast of the liquid crystal display element.
(4) To exhibit a nematic phase of liquid crystal composition within a wide temperature range, for the purpose of expanding a temperature range of an environment where the liquid crystal display element is used.
(5) To lower a threshold voltage (Vth) of the liquid crystal composition, and to reduce a temperature dependency (xcex4) of the threshold voltage of the liquid crystal composition, for the purpose of reducing the size of a battery which is a power source for driving the liquid crystal display element.
(6) To exhibit a high stability of the liquid crystal composition to heat and ultraviolet rays, for the purpose of preventing the deterioration of the liquid crystal composition to semi-permanently maintain a display quality of the liquid crystal display element.
(7) To reduce a frequency dependency of a dielectric anisotropy (xcex94xcex5) in a low temperature range, i.e., to exhibit a constant value of xcex94xcex5 in a higher frequency range (a large value of F10 which will be mentioned below), for the purpose of enabling the display at a low temperature range (xe2x88x9210xc2x0 C. or lower).
As the liquid crystal compositions for use in the STN mode having a relatively low threshold voltage and a relatively good temperature property at the threshold voltage (i.e., a small temperature dependency), there are mentioned compositions described in Japanese Patent Application Laid-Open Nos. 300582/1995, 300585/1995, 300584/1995, etc.
These liquid crystal compositions disclosed therein, however, have a drawback that they are poor in the stability to heat and ultraviolet rays, as shown in comparative examples of the present application. In addition, WO 96/11897 aims to provide a novel liquid crystalline compound having a large dielectric anisotropy and a remarkably low viscosity as a liquid crystalline compound for a low voltage in the various modes including the active matrix mode and the STN mode, and a liquid crystal composition containing the same. Herein, there is disclosed a composition containing a compound having xe2x80x94CF2Oxe2x80x94 as a bonding group and 3,5-difluoro-4-cyanophenyl as a terminal group (Composition Examples 19 to 22). Composition Example 19 disclosed herein is similar to the composition of the present invention, but as shown in the comparative example of the present application, it still has the drawbacks of a bad steepness, a high threshold voltage and a large temperature dependency at the threshold voltage.
As mentioned above, the various liquid crystal compositions have been investigated, but as for the liquid crystal composition for the STN display mode of the outdoor use, it is now required to satisfy the properties of the above paragraphs (5) and (6) in addition to the properties of the above paragraphs (1) to (4).
Furthermore, in the use for a mobile phone and the like, it is intended to perform a high duty for the purpose of expansion of displaying capacity. Accordingly, an actual driving frequency becomes higher and higher. As the driving frequency becomes higher, the movement of the liquid crystalline molecules at a low temperature cannot follow the change of the frequency of voltage, so that some serious problems rise, for example, a ghost phenomenon occurs, and the display is impossible owing to the decrease of contrast. In order to avoid such an insufficient display, there is more and more strongly required the liquid crystal material having an excellent frequency dependency (a small frequency dependency) of xcex94xcex5 at a low temperature range mentioned in the above paragraph (7), i.e., the liquid crystal material capable of maintaining a constant value (of xcex94xcex5) up to a higher frequency.
The specification of Japanese Patent Application Laid-Open No. 251186/1998 discloses, in Examples 46, 47 and 49, the liquid crystal compositions extremely similar to those of the present invention, but these compositions do not solve the problem of the present invention that a frequency dependency of xcex94xcex5 in a low temperature range mentioned in the above (7) is to be improved as shown on the comparative example below.
The problem to be solved by the present invention is to provide a liquid crystal composition which satisfies general properties required for an STN mode and which especially has a low threshold voltage, a small temperature dependency thereat, a high stability to heat and ultraviolet rays, and a small frequency dependency of a dielectric anisotropy (xcex94xcex5) at a low temperature.
The present inventors have extensively investigated compositions using various liquid crystalline compounds in order to solve the above problem, and as a result, it has been found that the problem can be solved by a composition comprising two specific components of a first component and a second component, and further excluding a specific component as another component therefrom. Thus, the present invention has now been achieved on the basis of the finding.
The liquid crystal compositions of the present invention will be described by the following paragraphs (1) to (4).
(1) A liquid crystal composition comprising:
a first component comprising a compound represented by the general formula (1), and
a second component comprising at least one compound selected from the group consisting of compounds represented by the general formulas (2-1), (2-2), (2-3) and (2-4), without any liquid crystalline compound having a dielectric anisotropy value (xcex94xcex5) of 5 or more and three or more six-membered rings, as another component: 
wherein R1 represents an alkyl group of 1 to 10 carbon atoms in which one methylene group may be replaced by an oxygen atom or xe2x80x94CHxe2x95x90CHxe2x80x94; R2, R3 and R4 each independently represent an alkyl group of 1 to 10 carbon atoms in which one methylene group may be replaced by an oxygen atom or xe2x80x94CHxe2x95x90CHxe2x80x94 and one or more hydrogen atoms may be substituted by fluorine atoms; R5 represents an alkyl group of 1 to 10 carbon atoms in which one methylene group may be replaced by an oxygen atom; R6, R7 and R9 each independently represent an alkyl group of 1 to 10 carbon atoms; R8 represents an alkyl group of 1 to 10 carbon atoms in which one methylene group may be replaced by an oxygen atom; A1, A2, A3 and A5 each independently represent a trans-1,4-cyclohexylene group or a 1,4-phenylene group; A4 represents a 1,4-phenylene group or a 1,4-phenylene group substituted by one to four fluorine atoms; Z1 represents xe2x80x94Cxe2x89xa1Cxe2x80x94 or a single bond; m and n each independently represent an integer of 0 or 1; and X1, X2, X3, X4 and X5 each independently represent a hydrogen atom or a fluorine atom.
(2) A liquid crystal composition according to the above-mentioned (1) wherein the contents of the said first component and the said second component are 5 to 50% and 10 to 85%, respectively, with respect to the total weight of the liquid crystal composition.
(3) A liquid crystal composition according to any one of the above-mentioned (1) and (2), which further contains a third component comprising a compound represented by the general formula (3) 
wherein R10 represents an alkyl group of 1 to 10 carbon atoms in which one methylene group may be replaced by an oxygen atom or xe2x80x94CHxe2x95x90CHxe2x80x94; A6 represents a trans-1,4-cyclohexylene group, a 1,4-phenylene group or a 1,4-phenylene group substituted by one to four fluorine atoms; Z2 represents xe2x80x94COOxe2x80x94, xe2x80x94C2H4xe2x80x94 or a single bond; and X6 and X7 each independently represent a hydrogen atom or a fluorine atom.
(4) A liquid crystal composition according to the above-mentioned (3), wherein the contents of the said first component, the said second component and the said third component are 5 to 50%, 10 to 85%, and 5 to 40%, respectively, with respect to the total weight of the liquid crystal composition.
A liquid crystal display element of the present invention is shown by the following paragraph (5).
(5) A liquid crystal display element comprising the liquid crystal composition according to any one of the above-mentioned paragraphs (1) to (4).